The Hydra is an ancient serpentine marine monster, a fearsome beast in Greek mythology who grew two heads for each one removed by a valiant warrior. Its real life equivalent, a freshwater polyp no longer than a centimeter (0.4 inches), is far less frightening but still quite remarkable – it is able to regenerate its entire body from any tiny fragment of itself. A new study published in the Philosophical Transactions of the Royal Society has revealed that this little critter can actually modify its genetic coding to suit its needs, sending certain cells into overdrive in order to perform additional functions.
Hydra belong to a large grouping (phylum) of animals called the cnidarians, which also includes jellyfish, sea anemones and corals. Their distinguishing features are a set of cnidocytes, cells that are specialized for prey capture. Hydra are particularly unusual, though, in that they do not seem to die. These ageless hunters spend much of their life motionless, moving rapidly only when their prey – mostly tiny plankton – wander past. When they do need to move, however, they do this by somersaulting, or “looping,” bending themselves over and using their tentacles to grip their prey or a surface.
Their regenerative abilities have long puzzled biologists, who cannot understand how they can do it even when they lack neurons or even stem cells. In order to investigate this, a team of researchers at the University of Geneva looked at the way Hydra express genes – basically whether they are switched on or off.
Comparing Hydra that did not possess stem cells to those that did, the team noticed a difference in the behavior of their epithelial cells, those that make up the creature’s outer tissue coating. “We identified 25 overexpressed genes in epithelial cells,” said Yvan Wenger, coauthor of the study, in a statement. “Some of these genes are involved in diverse nervous functions, such as neurogenesis or neurotransmission.”
Image credit: Hydra viridissima. Mikrofoto.de/Wikimedia Commons; CC BY-SA 3.0
It appears that when Hydra become entirely depleted of stem cells, and are no longer able to produce new neurons and grow new tissue, the organism induces an “overdrive” function in some of its epithelial cells. These cells then alter their genetic program to fill in this functional gap accordingly; in effect, the Hydra modifies its own gene expression. The new “enhanced” epithelial cells have heightened sensitivity to their surroundings, allowing the Hydra to compensate for its lack of neurons.
At this point, the specifics of how these epithelial cells are able to overexpress these genes remains unknown. However, this ability to alter its cellular functions in this way is unique, and the authors describe it as having “cellular plasticity.”
These creatures are structurally quite primitive, containing none of the higher functions of many other animals. Considering this, the authors think that their regenerative ability may be extremely ancient, something that evolved at a time when more advanced, specialized cells – those responsible for vision, for example – hadn’t developed yet. This plasticity was required by Hydra and their ancestors to mimic functions that other, more complex organisms already possessed.